The present invention relates to frequency modulation of optical signals, and, more particularly, a frequency modulated laser that can be modulated at almost unlimited modulation rates.
Modulation of information onto optical signals is well-known in the art. One modulation technique used for modulating optical signals is frequency modulation, which allows an improvement in Signal to Noise Ratio (SNR) by utilizing a transmission bandwidth that is larger than the signal bandwidth. Of key interest in frequency modulating optical signals, is the wide bandwidth available for frequency modulation due to the high carrier frequency in optical signals used for information transmission. Typically, communication systems offer bandwidths that are about 10% of the carrier frequency. A 1.5 xcexcm optical system, with a carrier frequency of 200,000 GHz, has, therefore, 20,000 GHz of instantaneous bandwidth. This provides the ability to modulate data at extremely large data rates onto the optical signal.
To take advantage of the wide bandwidths available at optical frequencies, apparatus and techniques must be used that provide for high modulation bandwidths. One approach is to couple an optical laser source with an external optical frequency modulator (i.e., a phase modulator with an appropriate driver). External modulators may require complex traveling wave structures that match the propagation velocities of the modulating RF field and the optical carrier. Another, less complex, approach is to directly frequency modulate the optical laser source.
Frequency-modulated lasers are well-known in the art. A conventional technique for providing a frequency-modulated semiconductor laser is to directly modulate the drive current of a Distributed Feedback (DFB) laser. Modulation of the drive current provides about 150 to 300 MHz of frequency shift per milliamp of drive current for commercial DFB lasers, yielding a maximum frequency swing of 10-20 GHz. This approach has two significant drawbacks. First, modulation of the drive current produces significant amplitude modulation, which is undesirable for FM communication systems. Second, an upper modulation rate of only about 20 GHz is achievable, due to semiconductor device limitations.
Another approach for providing a frequency-modulated laser is shown in FIG. 1. In FIG. 1, a laser cavity 10, as defined by the partial mirrors 15 at each end, comprises an optical gain section 11 combined with a phase modulator 13 of finite length. The partial mirrors 15 provide the required reflectivity for the lasing effect. The gain section 11 is typically a laser diode and the phase modulator 13 is a section in which the index of refraction is changed by the application of either a voltage (Stark Effect) or an injection current. The inclusion of the phase modulator 13 within the laser cavity 10 converts any phase modulation produced by the phase modulator into frequency modulation. It does this by changing the effective optical cavity length, which changes the laser oscillation frequency accordingly.
Semiconductor lasers which provide for an active region and a phase modulation region are described by Emura et al., in U.S. Pat. No. 5,325,382, xe2x80x9cMethod and Electrode Arrangement For Inducing Flat Frequency Response In Semiconductor Laser,xe2x80x9d issued Jun. 28, 1994, and Kawamura, in U.S. Pat. No. 5,481,559, xe2x80x9cLight Modulator Integrated Light-Emitting Device And Method Of Manufacturing The Same,xe2x80x9d issued Jan. 2, 1996. Emura et al. describe the application of a modulation current to both the active region and the phase modulation region to achieve a relatively flat frequency modulation response. Kawamura describes the provision of a ground electrode between the active region and the phase modulation region to isolate the electric field in the active region from the modulating electric field in the phase modulation region.
In the frequency-modulated laser shown in FIG. 1, the instantaneous frequency of the laser is given by:                               f          ⁡                      (            t            )                          =                                            1                              2                ⁢                                  xe2x80x83                                ⁢                π                                      ⁢                                          ⅆ                φ                                            ⅆ                t                                              =                                    f              0                        +                                                            Φ                                      0                    ⁢                    m                                                                    2                  ⁢                                      xe2x80x83                                    ⁢                  π                  ⁢                                      xe2x80x83                                    ⁢                                      T                    c                                                              ⁢                                                                    sin                    ⁢                    c                                    ⁡                                      (                                          ω                      ⁢                                              xe2x80x83                                            ⁢                                                                        T                          p                                                /                        2                                                              )                                                                                        sin                    ⁢                    c                                    ⁡                                      (                                          ω                      ⁢                                              xe2x80x83                                            ⁢                                                                        T                          c                                                /                        2                                                              )                                                              ⁢                              sin                ⁡                                  (                                                                                    ω                        m                                            ⁢                      t                                        +                    ϑ                                    )                                                                                        (        1        )            
where xcfx89 is the modulation frequency, Tp is the round-trip transit time for light in the phase modulator, Tc is the total round-trip transit time (phase modulator section+gain section) and "PHgr"0m is given by:                               Φ                      0            ⁢            m                          =                                            2              ⁢                              xe2x80x83                            ⁢              π                        λ                    ⁢          Δ          ⁢                      xe2x80x83                    ⁢                      n            0                    ⁢          2          ⁢                      xe2x80x83                    ⁢                      L            p                                              (        2        )            
where xcex94n0 is the electrically-induced change in the index of refraction, Lp is the length of the phase modulator, and xcex is the wavelength of the light generated by the laser.
Because Tc will always be larger than Tp, the term in the denominator will approach zero first, and produce the resonant response shown in FIG. 2. The resonant frequency of 1/Tc prevents the use of this prior art modulator for applying frequency modulation in the vicinity of the resonant frequency, and thus limits the bandwidth of the modulator. Hence, this prior art modulator is not suitable for high-fidelity, high bandwidth analog optical links.
If the total round-trip travel time for the laser cavity, Tc in equation 1 above, were identically equal to the round-trip travel time within the phase modulation section, Tp in equation 1 above, the two sinc terms in equation 1 would cancel, and the frequency response would be flat. Creation of a frequency-modulated laser where Tc equals Tp would provide a FM laser with a perfectly flat response over an almost infinite bandwidth.
There are two ways to make Tc equal Tp, one asymptotic, one absolute. The asymptotic approach is to make the length of the gain section very small compared to that of the phase modulation section, so that Tc asymptotically approaches Tp. The absolute approach is to make the gain and phase modulation sections coincident, so that the length of the section in which no phase modulation occurs is zero.
One structure in which the gain and phase modulation sections are longitudinally coincident is the twin-guide structure developed by Amman et al, as shown in the laser device 300 of FIG. 3. In spite of its name, the twin-guide structure is actually a single-guide device. The laser device is split into two sections, one being the phase modulation section 310 or tuning zone, the other the gain section 320 or active zone. The relative indices of these two sections are both higher than that of the outside cladding regions 330, so that the optical mode is confined in the center, just as it is in an optical fiber. A thin doped center layer region 340 is used to forward-bias the gain section 320, so that the medium is inverted (for gain), and to back-bias the phase modulation section 310, so that the index of refraction within the laser cavity can be controlled, either by means of the electro-optic effect, or by a band-edge or quantum-confined Stark effect. A Bragg grating 350 etched into a substrate 370 provides internal reflection within the structure for feedback. The device shown in FIG. 3 actually uses forward bias in the phase modulation section in order to get large frequency excursions. However, others have made similar devices that use a reverse bias to control the dielectric constant (e.g. as described by Wolf et al., xe2x80x9cModulatable Laser Diode For High Frequencies,xe2x80x9d U.S. Pat. No. 5,333,141, issued Jul. 26, 1994.).
In operation, a laser current is applied at the laser electrode 361 to provide a constant current to the gain section 320 to provide the optical gain required for laser operation. A modulation current is applied at the modulation electrode 363 to control the index of refraction with the phase modulation section 310 for modulating the optical frequency of the laser structure 300.
The performance of the twinguide laser device 300 is limited either by the finite capacitance of the structure, if the phase modulation section is operated in the reverse bias mode, or by the recombination time of the injected carriers, if operated in the forward bias mode. For reverse-biased structures, the phase modulation section 310 and the center layer region 340 act as a capacitor, thus providing a finite capacitance. This capacitance limits the frequency of the modulation signal that can be applied to the modulation electrode 363. The capacitance may be reduced by reducing the size of the phase modulation section 310, but this will reduce the maximum frequency excursion provided by the device. Forward-biased devices are capable of relatively large frequency swings, but have a maximum modulation rate of 200 MHz, due to the recombination time of the injected carriers.
In addition, the modulation frequency provided by the twinguide laser device 300 is limited by the radio frequency characteristics of applying a high frequency signal at the center of the modulation electrode 363. The modulation electrode 363 is a relatively long and narrow metal layer, such that a high frequency signal applied at the center of the electrode will propagate away from the center of the electrode towards the ends of the electrode and then reflect from those ends back to the center. The electric field that results from the applied high frequency signal will, therefore, not cause an equal change in the index of refraction within the laser cavity simultaneously. As the frequency of the modulation signal becomes higher, or the length of the device becomes longer, the radio frequency propagation effect reduces the modulation effectiveness of the twinguide laser device 300, when operated without a transverse-driving electrode. As described below, the transverse-driving electrode of the present invention provides improved modulation effectiveness.
Existing apparatus and methods for generating frequency-modulated light have limitations that restrict the modulation of light at extremely high modulation frequencies. The speed of direct modulation in a laser diode is limited by photon lifetimes and the lifetimes of the injected carriers. External frequency (phase) modulators that use changes in the index of refraction must employ traveling wave structures that match the propagation velocities of the RF and optical fields. The higher the modulation frequency, the more difficult it is to achieve this velocity match.
Therefore, there still exists a need in the art for apparatus and methods that provide for very high bandwidth frequency modulation of optical signals.
Accordingly, it is an object of the present invention to provide apparatus and methods for providing high bandwidth frequency modulation of optical signals.
High modulation rates for frequency modulated lasers are provided by embodiments of the present invention by making the gain and phase modulation sections of the laser cavity longitudinally coincident and by changing the index of refraction within the phase modulation section uniformly and simultaneously in time. The index of refraction is changed by passing the modulating field over a laser cavity transversely, so that the entire length of the laser cavity sees the same time element of the modulating field at the same time. The laser cavity must be either partially or totally filled with a material that will change its index of refraction in the presence of an electric field. Note that, according to the present invention, the index of refraction throughout the entire laser cavity does not have to be uniformly changed, but only that the index if refraction along an entire lengthwise portion of the laser cavity be uniformly and simultaneously changed. A traveling wave structure is used to apply the electric field to the laser cavity such that the radio frequency field in the traveling wave structure propagates perpendicularly to the direction of light propagation. The traveling wave structure does not need to match the propagation velocities of the light and RF fields, since the two fields propagate in orthogonal directions.
One embodiment of the present invention is provided by a frequency-modulated laser comprising: a laser cavity comprising an electrically-sensitive material, said laser cavity having a length dimension and width dimension, and said laser cavity producing laser light propagating substantially parallel to the length dimension of the laser cavity; and means for applying an electric field across said laser cavity, said electric field propagating in a direction substantially perpendicular to the direction of propagation of laser light within the laser cavity. The electric field is applied to change the index of refraction throughout the laser cavity substantially simultaneously and homogeneously with changes in the electric field. The laser cavity may be provided by the active region within a laser semiconductor structure or by a laser cavity within a pumped laser.
Another embodiment of the present invention is provided by a method for frequency modulating a laser light signal with an electrical signal, said method comprising the steps of: providing a laser cavity with a length and width, the laser cavity providing a lasing condition; producing laser light within the laser cavity, the laser light propagating in a direction substantially parallel to the length dimension of the laser cavity; maintaining the lasing condition with energy applied to a gain medium within said laser cavity; applying the electrical signal across said laser cavity to produce an electric field; the electric field uniformly and simultaneously changing the index of refraction along the entire laser cavity length in proportion to the amplitude of the electric signal; and transmitting the laser light out of the laser cavity to provide a frequency-modulated laser light signal.
Another embodiment of the present invention is provided by a frequency-modulated laser having a laser cavity fabricated of an electrically-sensitive material and a traveling wave structure disposed to apply a radio frequency field across the laser cavity, the radio frequency field propagating in the traveling wave structure in a direction substantially perpendicular to the direction of laser light propagating in the laser cavity. The changes in the radio frequency field change the index of refraction throughout the laser cavity substantially simultaneously and substantially uniformly. The laser cavity may be provided in a laser semiconductor structure, a pumped laser, or other laser structures known in the art. Preferably, the traveling wave structure comprises a transmission line and the laser cavity is disposed between the conductors of the transmission line such that the width of the conductors is greater than or equal to the length of the laser cavity at the point where the radio frequency field is applied across the laser cavity.